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Investigating the Slip History of the Multi-Stranded Patton Bay Megasplay Fault System, Montague Island, Alaska

Session: Cryptic Faults: Assessing Seismic Hazard on Slow Slipping, Blind or Distributed Fault Systems II

Type: Oral

Date: 4/21/2021

Presentation Time: 02:15 PM Pacific

Description: 

We investigated the southwestern end of the Patton Bay megathrust splay fault system on Montague Island, Alaska, to map 1964 surface rupture, characterize fault rock and off-fault deformation and evaluate linkages to offshore fault strands. The Patton Bay fault ruptured to the surface in the 1964 M_w9.2 Great Alaska earthquake, contributed to tsunami generation and displaced shorelines up to 10 m vertically. We collected new LiDAR data over this region to identify both the “Strike Creek” fault strand that ruptured in the 1964 earthquake and the “Deception Creek” fault strand 2 km to the northwest. Plafker (1967, USGS PP543-G) mapped both faults and found no evidence of surface rupture of the Deception Creek strand during the 1964 earthquake. During our fieldwork, we located the Strike Creek strand in forested uplands as one or two fault traces with NW-side-up scarps up to 1-m high. Tilted trees, open fissures, and left-transtensional fractures also defined the scarps. On the shoreline and beach bluffs, we were unable to precisely locate the fault in bedrock, despite excellent exposure and Plafker’s (1967) careful documentation of this part of the rupture. Similar Paleogene bedrock sandstone beds are on both sides of the fault and the relationships do not require significant offsets. These observations are consistent with the hypothesis that the 1964 rupture was the first rupture of this strand of the fault. Despite the lack of evidence of 1964 rupture of the Deception Creek strand, we documented a large NW-side-up scarp up to ~15 m tall, a well-developed fault gouge zone up to 1-m wide, and an associated damage zone. This strand is co-linear with the offshore trace of the Patton Bay fault, which we mapped with multibeam sonar. We conclude the Deception Creek is the main strand of the Patton Bay fault system on SW Montague Island, despite not rupturing in 1964. The different geomorphic expressions and geological characteristics of these two strands illustrate the variability of splay fault activation in megathrust events.

Presenting Author: Peter J. Haeussler

Student Presenter: No

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